I agree with I C on this matter.
1st: What is the year model, make and engine on your van.
2nd: What condition have you noticed that makes you want to override the electronic control of the radiator fan? Is the engine overheating at slow driving speeds or stopped idling? Is the A C system over pressuring, venting refrigerant because the radiator fans are not running?
Not knowing which model year you are referencing I looked at the radiator fan control circuitry for a 2010 and 2012 model year Grand Caravan. The wiring schematics were identical. It appears the TIPM controls a low fan speed relay and a high fan speed relay. I am guessing the TIPM receives communication signals from the PCM about radiator fan control. In the low fan speed relay load circuit there is a schematic box that identifies a resistor for providing an actual low fan speed. The high speed fan relay is wired directly through a fused circuit from the battery to the fans. Other than wiring connectors and the TIPM, the circuitry is not that complicated.
If you answer the basic questions we may be able to provide some guidance.
Here is the schematic I uploaded to my drop box that shows the inputs from the Temp sensor and the ECT
Dropbox - schematic radiator fan cooling.jpg